1. Field of the Invention.
The present invention relates to methods and aqueous acid compositions for reducing metal surface corrosion and the precipitation of metal sulfides from the aqueous acid compositions when acidizing a sour well or other similar location.
2. Description of the Prior Art.
Aqueous acid solutions are commonly used to treat oil or gas wells. For example, subterranean well formations are often contacted with aqueous acid solutions to increase the permeabilities of the formations whereby the production of oil and/or gas therefrom is increased. Aqueous acid solutions are also utilized to etch flow channels in the faces of fractures formed in the formations and to clean out perforations and tubular goods in wells.
When the well being treated is sour, i.e., the fluids produced contain a significant quantity of sulfide compounds, and particularly hydrogen sulfide, problems are often encountered as a result of the precipitation of metal sulfide such as iron sulfide from the aqueous acid solution. That is, dissolved sulfide ions and dissolved ferrous ions from ferrous sulfide scale and/or metal ions from the subterranean formation being treated react to form metal sulfides in the acid solution. The metal sulfides precipitate from the acid solution when the pH reaches a level greater than about 1.9. The pH of spent aqueous acid solutions used to treat subterranean formations is greater than 1.9, and therefore, metal sulfides dissolved in an unspent acid solution precipitate at some point before becoming spent. The metal sulfide precipitates can plug the treated formation and cause serious damage to the well.
Aqueous acid solutions are also used to remove scale from tubular apparatus such as heat exchangers, boilers and the like. If the scale contains metal and sulfide compounds, metal sulfide precipitates can form which interfere with the cleaning process.
While a variety of methods and compositions have been developed heretofore for preventing the precipitation of metal sulfides from aqueous acid treating solutions, there is a continuing need for improved acidizing methods and compositions which are relatively inexpensive and produce little or no metal sulfide precipitates.